Method of producing metal castings and cast products resulting therefrom



Patented Jan. 28, 1941 UNITED STATES PATENT OFFICE FROM Alexander C. Munro, Britannia Beach, British Columbia, Canada No Drawing. Application April 17, 1939,

Serial No. 268,340

8 Claims.

This invention relates to the production of metal castings, and to the castings so produced, and pertains particularly to the production of sound castings of high hardness and resistance to impact stresses.

One of the particular objects of the invention is to produce sound castings of high strength and hardness (i. e., high resistance to impact stresses and abrasion) in a process by which the customary reheating and quenching steps are eliminated, and improved castings are obtained at a lower cost.

A further object of the invention is to provide a method of producing metal castings involving a quenching procedure which subjects a cast object to rapid cooling while the object retains an important proportion of the heat imparted to it in the melting operation, and preferably while a significant proportion of the casting is composed W of metal in fluid condition.

In the following specific disclosure, the invention is applied primarily to the production of ferrous castings within a preferred carbon range of 130% to- 1.2% in the production of cast objects particularly useful as grinding media. The application of the present teachings to ferrous castings and particularly steels within the commonly accepted carbon content range, as well as to nonierrous alloys such as copper-nickel alloys, copper-silver, copper-aluminum, copper-tin, and also to ternary or more complex non-ferrous alloys will be appreciated.

The essential concept of the invention, as applied to the production of castings from ferrous and non-ferrous alloys exhibiting well-defined temperature-separated liquidus and solidus boundaries in their constitution diagrams, lies in the rapid quenching of castings made from such alloys from temperatures within the liquidussolidus boundaries or closely adjacent but below the solidus boundary, in the absence of any reheating step such as is commonly practiced in the heat treatment of steels. The present processis specifically applicable to 45 the production of steel grinding media, such as grinding balls and ball mill liners of the type employed in the metallurgical and other industries for fine grinding, as well as to other objects which are subjected to abrasion in use. In carrrying out the objects of the present invention, the molten metal alloy is cast into a mold and promptly after at least partial solidification oi the casting is attained, the casting is subjected to a rapid quenching treatment which will serve to temperature of the metal at a very rapid rate. The average temperature to which the casting is cooled from molten condition and at which the rapid quenching operation is carried out is below the liquidus temperature of the alloy of which 5 the casting is formed, but is not substantially below, and preferably at or above, the solidus temperature of such alloy. As applied to the production of steel castings, the casting is subjected to this rapid quenching while the average temperature of the casting is at a point on the constitution diagram of the metal which is below the liquidus line but not more than 100 F. below the solidus line, and more preferably at a temperature between the liquidus and solidus lines, which insures that the average temperature of the casting will be above the critical temperature of the alloy from which it is formed, at the time the casting is subjected to the quenching operation.

' When referring to solidus and liquidus temperatures in connection with a casting of any fixed composition, the temperatures indicated are those specifically fixed on the the Fe-C diagram by the per cent carbon in the steel, subject, of course, to the usual modifications resulting from the presence of other alloys such as Mn, Si, Cr, Ni, Cu, etc.

When a casting is allowed to cool in the mold from the casting temperature, a marked segregation of constituents of different carbon contents is experienced. As the alloy cools, the constituent first separated from the molten magma is that of minimum carbon content, and the grains of solid metal so produced grow by solidification of high carbon content material around the low carbon centers u'ntil finally that portion of the steel which is richest in carbon content will be last to solidify. This differential solidification often results in a dendritic structure in which the central portion of the dendrite is higher in iron content and lower in carbon content. 011 slow cooling below the solidus line (after complete solidification of the casting), the carbon content of the steel undergoes difiusion and the dendritic structure tends to disappear, accompanied by a gradual grain growth. In ordinary practice, the resulting large grain structure is considered unsuitable, in View of the fact that the resulting strength and resistance to abrasion are low, and it is common practiceto reheat the casting and subsequently quench the same from a temperature somewhat above the critical temperature of the metal, to produce a finer grained structure, but this treatment is for the'most part unsuited to the production of eutectoid and hypereutectoid steel cast- 55 ings by reason of the development of quenching cracks in the castings. p

when castings are produced according to the process of the present disclosure, Brinell hardnesses of in excess of 700 are readily obtained at carbon contents in the neighborhood of 1.0%, in the absence of the production of quenching cracks, while at the same time a more tough and dense structure is obtained, which is of high and sub- 10 stantially uniform hardness from the skin through to the center of the casting.

As a specific example of the application of the present invention to the production of steel grinding balls, I may take a steel containing approxi- 5 mately,1% carbon (along with small percentages of the common alloying constituents such as silicon, chromium, manganese, etc.), and melt the heat in an electric furnace to a temperature in the neighborhood of 2,900 F. (from 180 to 360 FR above the temperature on the liquidus line corresponding to a carbon content of 1.0%). The pouring is preferably done from ladles holding a, comparatively small quantity of melt and, as quickly as possible, the steel is poured into the 25 molds (which are conveniently made of sand by the snap method). Within approximately 30 seconds after casting, the wooden frame on the mold is removed and the mold dumped on the ground, or on a screen, and the balls then picked 30 up with a fork and thrown into a cold water quenching tank.

The total elapsed time between casting and quenching may vary, but is not usually more than one to three minutes and this total elapsed 85 time depends upon the cross-section of the casting, the pouring temperature, and the rate of cooling in the mold, and other conditions. In any event, the elapsed time between the actual casting and the actual quenching will be so re- 0 stricted that the average temperature of the casting will not fall much below the solidus temperature of the particular alloy of which the casting is formed, and, according to the preferred practice of the invention, will be so restricted 45 that the average temperature of the casting will be between the liquidus and solidus temperatures of the alloy.

It is preferable that the molding sand be sufficiently moist to allow eflicient binding, but

50 should not be too moist if a clean surface condition of the casting is desired. The quenching tank is sufliciently large to allow adequate cooling of the castings, and cold water is preferably added at a suflicient rate to prevent the tem- 55 perature of the bath rising more than 9 or 10 F. during the quench. The water is preferably introduced in such manner as to keep the bath in circulation, as by means of jets.

It has been observed, by actual test, by open- 60 ing the mold at 1, 2, 3, 4, and 5 minutes after casting, that a two-inch ball is still mushy (i. e., contains liquid steel) at three minutes, and is possibly partially liquid at four minutes.

. Castings produced according to the above proc- 0 es exhibit a very fine grain structure, and are very tough and free from quenching cracks. Furthermore, the structure is very dense and compact clear through to the center of the casting, which is of particular advantage (in the pro- 7 duction of grinding balls) in preventing disintegration of the ball when worn down to small sizes (the small ball sizes being generally more eflicient in fine grinding). The texture of the steel castings, i. e., the macroscopic appearance 75 of the steel on fracture is somewhat similar in grain size to that of a steel of identical composition which has been cooled in the mold, reheated for one to three hours at about 1,600 F., and then quenched. The microstructure of the quickquenched castings, however, is quite dissimilar 5 from ordinary steel, and shows evidence of the differentiation of carbon content in the crystals,

in particular, it shows small crystalline aggregates of iron of lower carbon content embedded in a matrix of crystals of iron of higher carbon con- 10 tent, and it is believed that it is this heterogeneity of grain structure which contributes largely to the properties of the steel, that is, hardness, toughness, and freedom from quenching cracks.

As a specific example of the performance of steel grinding balls produced according to the quick-quenching procedure of the present methad, in comparison with the best obtainable grade of cast white iron balls, a ball consumption of but 1.2 pounds per ton of ore was obtained with quick-quenched steel balls as against 1.7 pounds ball consumption per ton of ore with white cast iron balls under the same grinding conditions.

The process is applicable to the production of comparatively large castings, and in this application it is feasible to dump the whole mold (casting and all) into the quenching tank to avoid distortion of the casting prior to quenching, water jets being relied upon to wash away the sand from the casting in order to facilitate fast quenching.

While I have specifically described the production of castings from snap" molds, it will be appreciated that other types of sand molds,

' as well as metal molds or chill casts, may be alternatively employed. Furthermore, mechanical means may be employed to transfer the castingsd from the mold to the quenching tank if desire The invention is particularly useful in the 40 production of castings from iron-carbon alloys within the carbon range of 0.2% to 1.7%. In the production of grinding media, to which the process is particularly applicable, I find ironcarbon alloys of carbon content between .5% and 1.7% to be more useful, and those steels of between .6% and 1.2% carbon are particularly useful. The use of the present process on steels containing one or more alloying constituents such as chromium, manganese, and nickel, has been found practical, and the adaptability of the process to the treatment of stainless and tool steels will be apparent. The present process presents a major economy in quenching from the as cast condition, due to the saving in heat requirements over the customary reheating and quenching procedure. Furthermore, where further grain refinement or heat treatment is required, the uniform fine grained structure exhibited by castings produced according to the present process will enable a shorter reheating period to be employed for thorough redistribution of the carbon. 1

It will be appreciated that the quenching medium employed will depend upon the toughness and hardness desired in the finished casting, the maximum hardness being obtained where an adequate supply of cold water under suitable agitation is provided as a quenching bath, and increased toughness resulting when the quenching is performed in hot water, oil, or other milder quenching. media.

It will also be appreciated that the shorter the time interval between casting and quenching and the more rapidly the casting is quenched,

encased the finer will be the grain size produced and that variation in carbon content within the structure will also tend to be reduced under these conditions. Further, it is clearly recognized that variation in carbon content does occur in the structure of ordinary steels as for instance between the lamellae in the common pearlitic structure and in the component parts of the martensitic and sorbitic and other common steel structures, but the carbon variation produced by rapid quenching while part of the casting is still liquid is of a difierent nature to that found in the steel structures named.

I claim:

l. The method of producing steel grinding media, such as grinding balls and ball mill liners, from a ferrous alloy containing between 40.2% and 1.7% carbon, which comprises: pouring such an alloy in molten condition into a moldtoiorm a casting thereof; cooling said casting in the mold from a molten condition to an average temperature below the liquiolus temperature and above the critical temperature of such alloy, but not substantially below the solidus temperature of such alloy, and then quicltly quenching the so cooled casting in cold water, whereby said casting is subjected to the quenching operation while at least the interior central portion thereof is composedof metal in fluid condition.

2. The method set forth in claim 1, the average temperature of said casting at the time of quenching being not more than 100 F. below such soliclus temperature.

3. The method set forth in claim 1, the average temperature of said casting at the time of quenching being not less than such'solidus temperature.

4t. the method of producing steel grinding media, such as grinding balls and ball mill liners, from a ferrous alloy containing between 0.6%

' and 1.2% carbon, which comprises: pouring such an alloy in molten condition into a mold to form a casting thereof; cooling said casting in the mold from a molten condition to an average temperature below the liquidus temperature and above the critical temperature of such alloy, but not substantially below the solidus temperature of such alloy, and then quickly quenching the so cooled casting in cold water, whereby said casting is subjected to the quenching operation while at least the interior central portion thereof is composed oi metal in fluid condition.

5. The method set forth in claim l, the average temperature of said casting at the time of quenching being not more than 100 F. below such solidus temperature.

6. The method set forth in claim 4, the average temperature of said casting at the time of quenching being not less than such solidus temperature.

7. A cast steel product for use as a grinding medium, comprising a ferrous alloy containing between 0.2% and 1.7% carbon having a sound core substantially free from voids and having a microstructure characterized by the presence of small crystalline aggregates of iron of relatively lower carbon content embedded in a matrix of crystals of iron of relatively higher carbon content, as formed by casting such an alloy into a mold, allowing such casting to cool from molten condition to a temperature below the liquidus temperature and above the critical temperature of such alloy, but not substantially below the solidus temperature of such alloy, and then quickly quenching said casting whereby the casting is subjected to the quenching operation while at least the interior central portion thereof is composed of metal in fluid condition.

8. ll. cast steel product for use as a grinding medium, comprising a ferrous alloy containing between 0.6% and 1.2% carbon having a sound core substantially free from voids and having a mlcrostructure characterized by the presence of small crystalline aggregates of iron of relatively lower carbon content embedded in a matrix of crystals of iron of relatively higher carbon content, as formed by casting such an alloy into a mold, allowing such casting to cool from molten condition to a temperature below the liquidus temperature and above the critical temperature of such alloy, but not substantially below the solidus temperature of such alloy, and then quick- 1y quenching said casting whereby the casting is subjected to the quenching operation while at least the interior central portion thereof is composed of metal in fluid condition. 

